The porosity and air permeability of paper and board are important quantities in terms of the final use properties of the product. These quantities are additionally important in, for example, the coating of paper. Furthermore, in certain areas of final use these quantities should be as far as possible constant.
Conventional fillers, i.e. mineral products in particulate form, have the disadvantage that they increase permeability and porosity of the base paper. Thus, an increase of the proportion of filler also increases the air permeability of the paper. Air permeability is in general characterized using the so-called Gurley number, which indicates the time (e.g. in seconds) in which a predetermined amount of air penetrates the layer examined. The greater the Gurley number, the higher the air permeability resistance and the lower the porosity of the paper. For this reason, when conventional fillers are used, it is necessary to alter the composition of the fiber material of the base paper by, for example, increasing the amount of fines, when it is desired to increase the proportion of filler and at the same time to maintain an air permeability/porosity of a constant magnitude. However, the increasing of the degree of beating of the pulp weakens the optical properties of the product being manufactured.